Yes your instructor is right a balanced posture makes it very hard for uke to pull tori over. However being off balance does not always benefit uke. For example:- let us say that you are off balance leaning forward & your uke reaches up to the front of your gi to pull you down. In this situation it is possible to inflict a rather painful knee strike on uke. Blend with their attack (i.e. let them pull you towards themself) drop your center & knees first, crash down on top of them.